The present invention relates to a machine tool that machines plural surfaces of a work successively while replacing a tool in a tool magazine according to a program for machining and, more particularly, to a numerically controlled machine equipped with a machined surface change means such as an indexing device that rotates a work about an axis perpendicular to the axis of a spindle to change the machined surface.
Conventionally, machine tools such as machining centers machine plural surfaces of a work successively while replace a tool in a tool magazine according to a program for machining. Such a machine tool is described in Japan Published Unexamined Application No. 63-123646. In this known machine tool, a tool magazine and a spindle head are mounted integrally. When the tool is replaced, the head is moved vertically along a column to change the distance between the work and the tip of the tool. Various tools of different shapes and dimensions such as taps, drills, and reamers are held in the magazine. When the tools should be replaced, the magazine is rotated to index the tool. Then, the rotating mechanism is driven to rotate a tool pot so that the tool for the next machining process can be aligned with the tool for the present machining process. Subsequently, an exchange arm is driven to exchange the tools.
When the tools on this machine are exchanged, the spindle head is moved back into the top of the column to prevent the tool for the present machining process from interfering with the tool for the next machining process and to prevent the work from interfering with a peripheral device of the work. Therefore, it takes a long time to shift the present machining process to the next machining process. In addition, during this time, the work is not machined. Consequently, the efficiency of the machining the work cannot be improved.
If the tool magazine is mounted integrally with the spindle head and the machine tool itself is miniaturized, the tools cannot be exchanged without large movement. This makes no difference with a machine with a separate tool magazine on top of the column. This integrated small type of the machine tool is not thus made best use of.
Another conventional machine tool is also provided with an indexing device, and a work is positioned on an indexing table of the indexing device to machine. Since the work can be rotated by the indexing table, various surfaces of the work can be machined. When the work is rotated to direct the surface to be machined to the tool after one surface is machined, the work is rotated by the minimum angle to the position for the next machining process. The work thus takes the shortest route. However, this machine tool presents the following problems.
(1) When the work takes the shortest route, the spindle head of the machine tool needs to be retracted in order to prevent the work and the peripheral device from interfering with the tool attached to the spindle head. The retracting spindle head is moved into a given position irrespective of the range of rotation. Therefore, the tool attached to the end of the spindle head moves excessively. As a result machining time is wasted.
(2) When the absolute angular position of the surface of the work to be machined next is displayed, the present angular position of the work can rarely be known. Therefore, the operator frequently fails to know whether the work should be rotated counterclockwise or clockwise when the work takes the shortest route for the next machining process. Accordingly, when the work is rotated, the operator must retract the spindle head for safety whether the work rotates clockwise or counterclockwise. When every step of the machining process is checked manually in the trace mode or test mode, the spindle head may be shifted to an appropriate position, not based on an NC program, before the work is rotated by the indexing device. The spindle head must be subsequently moved such that the spindle head should not interfere with the work or other component whether the work is rotated counterclockwise or clockwise, because the direction of next rotation is not know. As a result, the machining time is wasted. If the spindle head is positioned after the direction of rotation of the work is incorrectly estimated, then the work may interfere with and damage the tool.
(3) When the work is machined while the work is controlled to take the shortest route, an overstroke may occur during operation, depending on the kind of the work, on the manner in which the work is attached to the indexing device, or the kind of the indexing device. Then, the work may collide with and break the machining table, a cable on the indexing device, or other component. To prevent these situations, sufficient care must be taken in creating a program for machining.